Further Notes on Natural Bracing

Introduction

Figure 1 (above): A lateral branch forming a brace across a bark-included junction (BI junction) in common alder (Alnus glutinosa (L.) Gaertn.). Note the large swelling at the bracing point, indicating this natural brace has probably been in place for decades.

I have been told the best British comedy comes from self-deprecation – putting yourself down to make others laugh. Unfortunately, I’m pretty rubbish at self-deprecation… as I am with many other things. I will, however, try to fit a little humour into these further notes on natural bracing, where possible.

I have written this article as an update for those that have attended one of my ‘fork talks’, in the UK, USA, Hong Kong, Singapore, Ireland, Sweden, Italy… or High Wycombe (the only venue I’ve done twice on consecutive days – which led to a strange “Groundhog Day” feeling on Day Two). This will obviously disenfranchise readers who have not attended any of my fork talks, for which I can only say what one says about jokes that were funny at the time: “You really had to be there”. 

To make up for any alienation, some further UK-based fork talks are planned in August 2017 – and, if popular demand remains high, there may be a few more, amongst quite a few European talks I have been invited to give in 2018. Having given this training on natural bracing to over 1,000 arborists/arboriculturists now, there will come a natural point where we all become naturally bored with lectures on natural bracing, and then these series of talks will come to a natural end. I have three academic papers to come out on this topic, and data for a fourth contributed by one of our BSc (Hons) students at Myerscough, showing that natural bracing persists in Tilia cordata Mill. more than it does in Acer platanoides L., so that’s a different and less exhausting way to disseminate these findings further. 

Natural bracing and the new model for branch attachment

Although originally my six-year-long PhD was to be focussed just on bark-included junctions in trees, I frittered a good three years of it away on sorting out a better model for branch attachment, as the prevalent one used in arboriculture was illogical and unworkable. The simplified outcome of this work is a new anatomical model based on two key features that typically provide most of the strength to a branch junction: the twisted dense wood formed under the branch bark ridge (BBR) and, when one branch or stem is growing much more quickly than the other to which it is joined, the faster growing branch occludes the base of the slower growing branch– a process that, if it continues, makes the base of the smaller branch into a knot. Figure 2 identifies key anatomical similarities and differences between a codominant branch junction and a branch-to-stem junction. 

Figure 2: Drawing of dissected branch junctions. A: Co-dominant junction. B: Branch-to-stem junction. Some features that these two junction types share: P: pith, B: the point at which the pith bifurcates, BBR: the twisted denser wood that forms under the branch bark ridge, which helps to hold both branch junctions together. The branch-to-stem junction has other features, however: C: the branch collar, the point where the base of the smaller branch is being occluded by the faster-growing stem, G: a zone of ‘grain capture’ at the base of the branch’s knot, where, if the branch grows faster, more of the wood grain is ‘captured’ into the branch, but if the branch grows slower, more of the wood grain diverts around the branch to supply sap to the upper stem (Kramer & Borkowski, 2004). 

It is only with a good understanding of this new model of branch attachment that the effect of natural bracing makes sense. What is being “turned off” when a branch junction is naturally braced is the production of the dense xylem under the BBR – it does not stop the process of the faster growing stem occluding the base of the slower growing branch. 

And, if you’re sceptical, you can go and look for yourself at some naturally-braced branch junctions and you will see the need for separating out these two attributes in a branch attachment model. This new model provides an important clarification – and we no longer need to conceive of imaginary wood grain arrangements under the bark of a branch junction – its anatomy is much more predictable and straightforward, whatever its diameter ratio. A branch bark ridge (BBR) (or not) and a knot (or not) are the key anatomical features we need to identify in a well-formed (or not) branch junction.  

Is it a natural brace or not?

I have tried to carefully define what a ‘natural brace’ constitutes: hard, physical contact formed above a branch junction that greatly restricts or prevents dynamic and gravitational forces acting at that branch junction. This definition cannot be dumbed down to a simpler rule: I have had a few people ask me – “So touching branches means you get a bark included junction formed below them?”. Not necessarily – no. First, many touching branches do not straddle a branch junction. Second, two branches touching each other may restrict a branch junction’s movement but not remove gravitational loading from it – especially when that junction is not vertically orientated. Third, it is very much a matter of ‘timing’: if a branch junction has been normally formed for many decades, a recently formed natural brace will not suddenly convert it to be a bark-included junction (BI junction). The effect of natural braces is not so simple that you can sum it up in one basic rule – it’s complex, involving multiple factors, such as gravitational loading, dynamic movement and the effect of the brace over time.

However, I can suggest a straightforward way to approach the idea of natural braces and BI junctions overall. We are mostly happy with the science that shows that stems and branches respond to the loading they experience (Telewski, 2006) – well, branch junctions are also ‘reading their load’ and responding to strain to form their different morphologies. Mastering natural bracing and junction anatomy makes you a better ‘reader’ of branch junctions, for it gives a much more complete picture of why you get very different forms of branch junction in different loading scenarios.

From several observations, I have noticed that some BI junctions are not associated with a natural brace, but with ‘natural movement restrictors’ that form physical obstructions very near to the branches arising from a vertically-orientated BI junction – natural swaying of those branches is prevented in these cases, but they can move a little and they are not braced. The ones I have seen involved branches that could only rattle around in a cage-like inner crown, as they bumped into other branches or stems if they moved as little as 10 mm or so. Although I have not found this to be common, perhaps this effect represents another cause of BI junctions, mopping up at least some of those not explained by the presence of natural braces (in my cohort survey, natural bracing explained the presence of 93.9% of non-bulged BI junctions). Specimens with this effect I found on my travels in Dublin, Buckfastleigh and High Wycombe.

I initially defined nine types of natural brace (Slater, 2016), and there may be a tenth formed in tropical regions by aerial root systems – an Australian I met recently offered to send me evidence of this tenth brace, which sounds highly plausible, having seen aerial rooting in Hong Kong’s urban forest previously. I would note that some arborists that have not attended the fork talks and are making contact with me have the conception that a natural brace is only when two branches or stems fuse together above a branch junction (this process of fusion can also be referred to as ‘anastomosis’, ‘inosculation’, ‘natural grafting’ or ‘self-grafting’, it would seem – perhaps all these terms for the same thing should be fused together too!) – but that is only one out of the nine/ten types of natural brace to be found in trees, and by no means the most common type of natural brace either (Fig. 3). 

Figure 3: Bark-included junctions (each labelled ‘BI’) and their associated natural braces (each labelled ‘NB’). A: Lateral branch pressing on a stem of common beech (Fagus sylvatica L.); B: Entwining stems in a grey alder (Alnus incana (L.) Moench); C: Fused branches in a sycamore (Acer pseudoplatanus L.). 

Is a bigger bulge at a BI junction better?

The loss of a natural brace set above a BI junction, either through pruning or natural processes of self-shading, usually causes the formation of bulges of wood at the two points below the seam of included bark, as those areas will experience quite extreme strain at the loss of the bracing they have relied upon for many years. The bulging is a sign of the tree enacting a repair upon a weakened part of its structure (Slater, 2016).

It logically follows from this that if the BI junction bulges for several years, and in the absence of internal cracks, the bulging branch junction is becoming stronger each year by the addition of this wood where the peak strains are being experienced in comparison with when it started with little to no bulge. Although it has previously been promoted to arboriculturists that BI junctions with ‘big ears’ are those we should most worry about in terms of their likelihood of failure (Mattheck & Breloer, 1994), this advice seems highly suspect now the association of natural bracing with BI junction morphology has been outlined.

Just down from my house there is a mature sycamore tree with its first bifurcation being a BI junction with a noticeably peaked bulge at its base (Fig. 4). Although it’s a big bulge which some arboriculturists would see as a trigger for some risk management work, it’s not logical to argue that the more wood that is formed at a branch junction then the weaker it is. From our own experiments, we found interlocking wood grain patterns and denser, stronger wood within such bulges (Slater and Ennos, 2015), implying that these bulges can form two branch bark ridges, either side of the seam of included bark that initially weakened the junction. In young and semi-mature trees, this repair process results in the formation of a cup-shaped branch junction (cup union). These pointy-bulges are a feature of large-grown mature trees where a natural brace was lost at a later stage in the tree’s growth cycle, but some forms of these more extreme bulges may prove to be relatively stable too, upon scientific examination. 

Figure 4: A large ‘peaked’ bulge formed at a BI junction in a sycamore (Acer pseudoplatanus), which has been in this form for more than fifteen years. A: view of the bulge in the plane of the bifurcation; B: view of the bulge perpendicular to the plane of the bifurcation. This ‘big eared’ BI junction is probably not an immediate danger if it has persisted in this form for so long, despite previous guidance to the contrary. 

Shockingly, from following some old advice without any scientific research to back it up, we may well have been culling out, pruning and reducing many large trees based on ‘big ears’ found at a BI junction in their structure, and such work may have been wholly unnecessary for risk management. These extreme forms of bulge need to be tested under scientific conditions to grade them with the load that they are meant to be bearing. I suspect, in the absence of internal cracks within the wood, many of these junctions will prove to be satisfactorily stable, with no need to enact major tree work. This is quite a turn-around to current arboricultural practice if so – I do not expect that to happen overnight, even when a scientific study is published on this topic – but I thought I would share this logical observation in this article, to ‘plant a seed’ for some that are receptive to thinking differently. 

What makes cup union shapes vary so much?

Cup unions are a form of repairing or repaired BI junction that one sees on a much more regular basis than the ones with peaked bulges. However, they also take different forms. In my notes, I direct tree surveyors to look at whether there are signs of recent rapid growth at the bulges either side of the branch junction, which would suggest that the cup union is still experiencing excess strain and having to react to those strains (Slater, 2016). If the ‘body language’ of the cup union is much calmer, with no sign of rapid growth in one section, it has probably achieved a successful repair. One should check for decay forming within a cup union, though – it’s still a potential defect and a court for diseases such as Phytophthora that like the damp conditions a cup union often offers.

In a few venues, when we’ve looked at a cup union or two, I’ve been asked the question “Why is one side of this cup union higher than the other?”  Again, this is about the tree’s tissues experiencing strain and then responding – once one side of a cup union responds and gets higher than another, it is likely to take more of the tensile strain acting across the junction, and grow more on that side than the other side, unless the loading to the junction changes again. I have found lop-sided cup unions to be very common (Fig. 5), which I put down to eccentric loading of tension and torsion across the branch junction which favours the development of one side over another. This observation probably warrants a science-based study to support it. 

Figure 5: A cup union which has recovered much of the strength of a normally-formed branch junction. Note that one side of the ‘cup’ has formed higher than the other (white arrows): here, unequal strain levels are probably the direct cause of greater formation of wood under the BBR on one side than on the other side of this branch junction. 

Why is this unbraced bark included junction not bulging?

Here’s another rarity I have found only a few times so far (Edinburgh, Florence & Lancashire), showing that the interpretation of junctions in trees is potentially complex. Where more than two branches or stems arise from a junction, and any natural braces are long gone due to self-shading or pruning, you can get all but one of the branch junctions forming good branch bark ridges, and then one that does not bother and remains a BI junction (Fig. 6). Why would that happen? 

Figure 6: A quintfurcation in a mature sycamore (Acer pseudoplatanus), where four of the unions have good BBR formation, but the one on the right here is just a long bark-included junction with no obvious BBR. Overall, the cross-section of this multi-stemmed tree would be a horseshoe-shape, with an ‘opening’ at the single remaining BI junction. However, all five stems are joined by at least one BBR to another stem, so the tree has efficiently not bothered producing that fifth BBR, probably because it would be surplus to its structural requirements.

Again, the simple answer I can offer is that these branch junctions are ‘reading their load’ – once the other junctions in the circle of multiple tree stems have formed good connections with each other, there is the potential for very little loading to be received by the remaining one BI junction. Essentially, this leads to the formation of a "horseshoe" or arc of good connections between these stems (in cross-section or ‘plan view’) with one bark-on-bark connection remaining where BBR material has not been stimulated to grow. My instinct would be to say: “probably stable” to the BI junction in the tree shown in Figure 6 – but perhaps some research could investigate these multi-stemmed tree forms in more detail and determine if they are somewhat structurally flawed or not. 

What species are prone to bark-included junctions?

Well, in the UK, I have the benefit of 348 responses to a questionnaire, run when I delivered my first series of eleven ‘fork talks’ – so Table 1 gives some idea of what practitioners find as species where failures at BI junctions are most common. 

Table 1: Types of tree considered by 306 respondents to be prone to BI junction failure, in order of frequency of being named as problematic in an industry-based questionnaire. Only genera with more than ten mentions have been included (>3% of respondents). Where species or cultivars within a genus were referred to seven or more times (>2% of respondents), their frequency is provided on the right side of the frequency column, under the relevant genus.

Unfortunately, that data is nothing like a comprehensive trawl through the trees one might find planted in the UK – rather, it represents commonly-found trees that these respondents have personally witnessed having BI junction failures. It is interesting what is not on this list too – like hawthorn (Crataegus monogyna Jacq.) and yew (Taxus baccata L.), which both regularly contain BI junctions associated with natural braces, but tend to be smaller trees (or large shrubs) and often retain their natural braces for lengthy periods without shading them out.

What is obvious from my survey of a couple of thousand trees is that bushy, multi-stemmed tree forms and fastigiated trees are more likely to lead to branch-to-branch and stem-to-stem interactions, and therefore would be more prone to BI junction failure in general. Add to that the effects of fast growth at the semi-mature stage and a tendency to self-shade out one’s own branches, and this implies that trees more prone to BI failure would be Norway maple, willows, poplars, oaks… Hold on a sec, they are all listed in Table 1… Coincidence? I don’t think so! The creation and failure rate of BI junctions is somewhat predictable from a tree’s form – a self-set, multi-stemmed Salix caprea L. stands a good chance of forming quite a few natural braces in its crown due to its unruly branch structure, whereas trees forming only a single trunk and having nicely spaced small lateral branches that do not compete as leaders – like your average plantation Sitka - are not going to develop BI junctions readily.

However, even in the conifers, there are those prone to natural bracing – yew, as mentioned above, cedar (I have seen some fantastic examples of natural bracing in Cedrus spp. when giving talks at different venues: Askham Bryan, Dublin, Wokingham, Florence) and scale-needled conifers – especially Leylandii (Table 1). Once an arborist or arboriculturist has a strong conception of the effects of natural bracing, it gives a rational explanation as to why bark-included limbs of Leylandii regularly fail in the UK’s regular winter storms (Figure 7). 

Figure 7: With a magic lens, we can see inside a Leylandii (X Cuprocyparis leylandii) and (ignoring the ubiquitous pigeon nests and associated poop) we can see the development of natural bracing in the tree, the formation of an associated bark-included junction, the loss of the natural braces through self-shading, and then potential failure at the junction as it starts to bulge due to the strains it is now experiencing during wind movement. In many Leylandii growing near to you, this process is happening – but rest assured that it is probably only a proportion of Leylandii that suffer these BI junction failures.

Modelling the failure of BI junctions

After I wrote my course notes in June 2016 to support the initial eleven fork workshops in the UK, I got the chance to amend them in November of the same year. This revision of the course notes was useful, as I’d had a few thoughts since the initial version, I had made a few further observations, there were a few ‘typos’ to remove and one delegate had usefully suggested the use of the term ‘crossing branches’ which was important to adopt for better clarity of explanation in a couple of sections of the notes. My thanks to that contributor, whose name we could not retrospectively find out – but let me know if it was you!

In some talks, I did point out to attendees that BI junctions formed at or near the base of trees may present less of a hazard compared with those formed higher up in trees: an observation I had made many years ago, but neglected to place in the first version of my notes. The logic of this is simple – when the wind blows, flexure and displacement in a tree is much greater further up the tree (Fig. 8). Through the action of several factors (especially juvenile and more flexible wood formed higher in the tree and the effects of bending and mass dampening), not all the swaying motion of the upper part of an open-grown tree is transmitted down to the tree’s base (James, 2003). As a result, the upper stem and branches are displaced far more in high winds (potentially by a metre or more), whereas the base of the tree is not displaced very much at all (if the base has been displaced by a metre or more, the tree’s been uprooted!). It follows that BI junctions formed above head height in a semi-mature tree are more likely to fail during a wind event than an old BI junction formed at the base of a large mature tree. 

Figure 8: A: Simple diagram to outline the displacement in a tall upright tree under wind loading, identifying that movement in the wind is far greater higher up in the stem, towards the canopy, and that there is far less displacement at the tree’s base. B: Extrapolating from A, in general, it appears logical to argue that the failure of BI junctions that have lost or outgrown their natural braces may be significantly more likely higher up in the tree than near the ground, although failure of a BI junction is possible in all locations in my experience. However, this suggestion should be subjected to a scientific study, for the height of a BI junction above the ground may not be a relevant factor to its likelihood of failure. 

The logic is there for that differentiation by the height of the BI junction in the tree – but this effect (if it’s real) needs to be quantified by a scientific study so it can be considered as a factor in the assessment of BI junctions. Unfortunately, in the absence of such data, I could not add this factor into Table 1 or Table 3 of my notes, but in my revised version of the fork course notes, I do at least note this as a potential factor for investigation (Slater, 2016). 

Again, this would make a good research project – particularly if you have some tree motion sensors, tilt meters or accelerometers handy. Actually, I do have three accelerometers, so perhaps I can do this work as a follow-up study – if I get the chance. I have just three photographs of BI junctions that failed at the bases of trees – I have many more of BI junctions failing in the mid-crown of trees, two or more metres above ground level. Perhaps a large-scale survey of BI junction heights (failed and intact) after a strong storm may also find a significant association between BI junction height and their rate of failure, without needing complex instrumentation.

Conclusions

For anyone baffled by this article, you have either a) not attended one of my ‘fork talks’, b) not remembered much from one of my ‘fork talks’ or c) not really gained an understanding of natural bracing from the articles I have pasted up on Linked-In. My apologies.

But please do not blame me for your bafflement, as I am working hard to demystify branch junctions for arborists – I’ve come up with a new model for branch attachment, I’ve found the primary cause of bark-included junctions in trees, and I’ve written some notes (available from the Arboricultural Association’s online bookshop, by the way) on how we might manage trees a bit better in the light of my findings on natural bracing. I seem to have achieved a lot in a brief time (I told you I was rubbish at self-deprecation!), but there is a need for much more research into natural bracing and how it affects a tree’s biomechanical performance. 

I hope you found these additional notes interesting and I hope to contribute further research in this area myself, but one of the main purposes of sharing my findings on natural braces is to allow others to research in this area too. Hence this article – it highlights a few of my observations that we could usefully throw some proper scientific studies at, whoever is game to do that.

For me, one of the nicest aspects of finding the effects of natural bracing in trees has been that it has given me something new to teach people.  I am also enthusiastic about this topic because I can ‘read’ branch junctions so much better than before I carried out my research work and I’d like to pass that knowledge onto others. 

I can only hope that some of my other research into tree production, urban forestry and wood decay will come to bring something as new, shiny and exciting as ‘natural bracing’ into arboricultural training in the future – if it does, I’ll be sure to share it with you all. If it doesn’t, I will spend my time practising my self-deprecation skills, hopefully becoming one of the greatest self-deprecators that has ever lived. It might be hard to tell though! ;-)

Citations and related publications:

James K (2003) Dynamic loading of trees; Journal of Arboriculture 29, 165-171.

Kramer E M & Borkowski M H (2004) Wood grain patterns at branch junctions: modelling and implications. Trees: Structure and Function 18, 493-500.

Mattheck C and Breloer H (1994) The body language of trees: A handbook for failure analysis; London: TSO.

Slater D & Ennos A R (2015) Interlocking wood grain patterns provide improved wood strength properties in forks of hazel (Corylus avellana L.); Arboricultural Journal 37, 21-32.

Slater D (2016) Assessment of Tree Forks: Assessment of Junctions for Risk Management; 2nd edition; Stroud: Arboricultural Association.

Telewski F W (2006) A unified hypothesis of mechanoperception in plants; American Journal of Botany 93, 1466-1476.

This article first appeared in the AA's Arb Magazine, Autumn Edition, 2017